This invention relates to gear assemblies, and more particularly to planetary gear sets that can be used in rack-and-pinion systems and other such industrial applications.
Planetary gear systems are widely used in a variety of industrial environments. In such arrangements, the rotational input is usually in the form of a sun gear. A plurality of planetary gears are mounted about the sun gear for receiving rotational force from the sun gear through intermeshing of respective teeth. Conventionally, planetary gears are mounted on a rotating planetary gear area, and the output element is usually in the form of a ring gear.
A typical epicyclic gear or planetary gear system in addition to sun gear and a plurality of planet gears uses a ring gear provided with internal teeth. The sun gear is located in the carrier, with the planet gear engaged to the sun and ring gear going around both the planetary gears and the sun gear. The ring gear is typically engaged with all the planet gears. Thus each planet gear is engaged with both the sun and the ring gear, and to none of the other planets, while the ring and sun are each engaged with all the planets. The planets are all mounted to the shafts in a parallel relationship, which relationship would ideally be retained during rotation. Of these three sets of items, sun, planets mounted on the carrier, and the ring gear, one will typically be held fixed and the other to rotate, with power to rotate fed to one rotating component, at a given angular speed and torque, and power taken from the other rotating component at a changed torque and speed related linearly or inversely to the first by the gear ratio.
A common problem in all gear systems both planetary and non-planetary is misalignment of the two gears as their teeth mesh. When the axes of rotation of the gears are not perfectly parallel the partial contacts of the teeth cause expanding and contact stresses to one end of a tooth. Theoretically, potential power loss of the gear assembly output due to misalignment can be 30 percent or higher. The out-of-parallel condition causes significant problems in excessive wear, added friction, added noise, and higher stress in the gear teeth, which causes metal fatigue.
Another issue created in planetary gear assemblies with four or more planet gears is the load distribution between the planet gears. In order to better approximate uniform loading, one of the suggested methods is to allow elastic deformation of planet gear shafts and provide “flexible mounts.” Still another problem arises due to deflection of the carrier under load, which will introduce the most misalignment when the gears are subject to maximum load. At such time the carrier torsional deformation introduces the largest misalignment due to the deformation. A significant part of the large percentage of derating of all gears due to misalignment is directly attributable to this fact.
One of the solutions offered by the industry is to use a pair of spaced-apart rigidly connected plates to function as the planet carrier. The double-plate design allows to significantly reduce deflection of the planet shaft and misalignment. However, the conventional double-plate carrier designs are not well suited to the use of flexible mountings for the planets, which in turn makes them poorly suited to use of more than three planets. Also out-of-tolerance issues will tend to be aggravated by the stiffness of those designs
Another approach to the problem is shown in U.S. Pat. No. 3,303,713 issued to R. J. Hicks in 1967. According to the '713 patent, a sleeve is interposed between the gear and the shaft, upon which the gear wheel is located. The shaft has opposite end portions rigidly secured between the gear wheel and the carrier. The space between the gear wheel and the carrier is said to allow the shaft to flex to provide uniform loading. Hicks also teaches the shaping of the pin such that it is flattened on the two sides parallel to the radial axis of the sun and perpendicular to the tangential direction of the planet motion. The object of this design is to reduce the section modulus on that axis to allow larger deflections in that direction which better allows for load sharing, and also allows for better prevention of deflection in the radial direction due to centripetal forces.
While the system of the Hicks patent may work satisfactorily in certain environments, there exists a need for a gear assembly for use in high-load environment, such as for instance rack and pinion systems of a jack-up.